Pretreatment of molded polyolefins for electroless plating

ABSTRACT

The invention discloses compositions for and methods of treating the surface of molded polyolefins, more especially polypropylene, polyethylene and polybutylene, prior to the deposition of a metallic plate thereon in order to improve the adherence of the plate to the surface of the polyolefin substrate. The compositions comprise stable, aqueous microemulsions of the oilin-water-type containing as the principal active agents a mixture of white phosphorus and trichloroethylene.

United States Patent Grunwald et a1.

PRETREATMENT OF MOLDED POLYOLEFINS FOR ELECTROLESS PLATING John J. Grunwald, New Haven; Eugene D. DOttavio, Thomaston, both of Conn.

MacDermid Incorporated, Waterbury, Conn.

Filed: July 2, 1970 Appl. No.: 52,090

Related US. Application Data Division of Ser. No. 717,006, Mar. 28, 1968, Pat. No. 3,567,649.

inventors:

Assignee:

U.S.Cl ..117/47 A, l17/138.8 E, 117/160 R,

106/1, 204/30, 252/310 Int. Cl. ..B44d 1/092, B44d 1/02 Field ofSearch ..117/47 A, 130 E, 131, 160 R,

[ Mar. 7, 1972 [56] References Cited UNITED STATES PATENTS 3,489,585 1/1970 Miller ..106/287 3,556,956 l/197l Miller ..117/047 A X Primary ExaminerAlfred L. Leavitt Assistant ExaminerEdward G. Whitby Attorney-Steward & Steward, Merrill F. Steward, Donald T. Steward and Walter D. Hunter [5 7] ABSTRACT 6 Claims, No Drawings PRETREATMEN'I F MOLDED POLYOLEFINS FOR ELECTROLESS PLATING This application is a division of copending application Ser. No. 717,006, filed Mar. 28, 1968, now US. Pat. No.

The present invention relates generally to the pretreatment of surfaces of molded polyoletins for subsequent chemical plating of a metal on such surfaces, and is concerned more especially with novel pretreatment bath compositions, and processes of using such compositions, to enhance the adhesive strength of the resulting metal-to-plastic bond in the plated substrates.

The invention affords important practical advantages over compositions and processes known heretofore which have been used for similar purposes.

The successful application to commercial and industrial use of metal plated polymer articles hinges, to a great extent, on the strength of the metal-to-polymer bond. Properties such as the ability of the resulting composite of metal plate and polymer substrate to withstand extreme temperature changes and mechanical impact are directly related to adhesion of the metal to the polymer substrate. It is commonly known that the as-molded surfaces of most polymers do not ordinarily provide a satisfactory base for achieving a strong bond to a subsequently deposited film or plate of copper, nickel or cobalt, for example. Many procedures are known for modifying the surface of the substrate in one way or another to effect greater bonding strength between the substrate and metal plate. One of such known methods involves the use of white phosphorus dissolved in trichloroethylene. The plastic substrate surface is brought into contact with the phosphorus-trichloroethylene bath, usually by immersing the substrate in a suitable tank containing the solution, and is held in contact with the solution at an appropriate temperature for a sufficient period of time to effect the desired modification of the surface resulting in greater adherence of subsequently deposited metal film. The aforesaid process further involves careful rinsing of the pretreated substrate to remove excess phosphorustrichloroethylene solution adhering to the surface of the substrate before chemical plating, and for this purpose a water solution of dimethylforrnamide is used as a first rinse, which is then followed by a hot water rinse. The substrate is next treated in a nickel sulfate-ammonium hydroxide bath to form a nickel phosphide film which serves as a preliminary deposit for the subsequent electroless deposition of a heavier film of metal such as copper, nickel, cobalt, etc. The latter is usually supplemented by a further, still heavier, metal plate produced electrolytically.

The phosphorus-trichloroethylene system just described is quite effective in providing good bond strength of the resulting metal deposit on the substrate surface. The system, however, has disadvantages in that the phosphorus-trichloroethylene bath must be operated at temperatures of l50l60 F. to be effective, and at such temperatures there is substantial evaporation loss as well as a problem of providing adequate ventiliation to eliminate the danger of toxicity from fumes emanating from the bath. This can be and is controlled to some extent by the use of a water seal on the surface of the bath, but where automated plating operations are involved, the continuous entry of the articles into the treatment bath, and their subsequent removal, substantially interferes with the efiectiveness of the water seal. Along with the toxicity problem there is also one of fire hazard due to the phosphorustrichloroethylene fumes. Also, because of the low solubility of white phosphorus in trichloroethylene at normal ambient temperatures, bath compositions having sufficient high phosphorus concentrations to be effective cannot be manufactured and shipped in ready-to-use form but must be prepared at the use site. This is not only inconvenient but because of the extreme activity of white phosphorus, especially in oxidizing environments, there is a hazard to personnel in handling it. In addition, excess phosphorus-trichloroethylene solution remaining on the surface of treated articles upon removal from the pretreatment bath is not easily removed by ordinary water rinses, yet it is essential that this excess be removed in order to secure improved bond strength in the subsequently applied metal deposit. Accordingly it is necessary in the prior process described above to use a solvent, such as dimethylformamide, to eliminate most of the excess phosphorustrichloroethylene solution, and then to follow that organic solvent rinse with water rinses. It has also been found that there is an appreciable corrosive effect of the phosphorustrichloroethylene mixture upon most metals of the type that can be used practically in providing a tank or container for the pretreatment bath. The difficulty is not eliminated by resort to plastic coated treatment tanks, such as Plastisol" lined tanks, as there is a tendency of the lining to soften. Since most of the present electroless plating operations are carried out in plastic-coated tanks, the carryover of excess phosphorustrichloroethylene into those tanks, if adequate rinsing procedures are not employed, causes excessive plating out of metal on the surface of the tank itself from the electroless plating solution.

It is now been found and it is the basic concept of this invention that most of the foregoing difficulties of a phosphorustrichloroethylene pretreatment which have been mentioned above can be eliminated or substantially reduced while retaining the benefits of the better adhesion of metal film to plastic substrates which that system provides. Instead of using a true solvent-solute system of phosphorus dissolved in trichloroethylene as the major constituent of the treatment bath, the present invention employs a stable, aqueous emulsion of the oil-in-water type wherein phosphorus dissolved in a limited amount of trichloroethylene is emulsified, using suitable surfactants, so that water comprises the major constituent of the bath. It is found that such emulsion compositions can be prepared in ready-to-use form which can be shipped to a processing site and are ready for use without further preparation other than raising the temperature to increase the speed of the pretreatment. The optimum treatment temperature, moreover, is substantially lower than the heretofore required temperatures of l50l60 F. For example, the novel emulsion compositions operate quite effectively at temperatures on the order of l10-l20 F. It is also found that lower phosphorus concentrations are quite as effective as the much higher phosphorus concentrations required in the prior organic solvent bath. A further substantial advantage of the new bath composition is that of greater ease in removing excess pretreatment solution from the surface of the treated substrates prior to transfer to the electroless bath. In the novel process here disclosed a single cold 'water rinse is all that is required, thus-eliminating any organic solvent rinse. Other advantages of the present system over the earlier one include much lower evaporation losses, reduced toxicity and ventilation problems, substantially lower flammability hazards, lower concentrations, i.e., more economical operation and reduced corrosion of treatment tanks.

The invention is illustrated by the following example.

The novel pretreatment emulsion is best prepared in two steps in accordance with the following procedure:

STEP A An appropriate amount (e.g., 100 ml. for a 15 percent by volume composition) of trichloroethylene is mixed-with ml. water and 2.2 grams of white phosphorus is added. The mixture is heated to 1 10 with stirring until all of the phosphorus is dissolved.

STEP B The appropriatevolume of water to make up-the desired final solution (in this case 400 ml.) is heated while adding suitable surfactants, as for example ml. of Kamar-O and 5 ml. of Benax 2A1. Kamar-O is the trade name for-a wetting agent produced by Finetex Inc., East Paterson, New Jersey, comprising an anionic blend of alkylaryl .hydrotopes. Benax 2A1" is the trade name of Dow Chemical Company for sodium dodecyl diphenyl ether disulfonate. The resulting solution of Step B is added to that of Step A gradually with stirring while maintaining the temperature at approximately ll F. The resulting aqueous emulsion is an extremely tine dispersion and is highly stable so that it may be stored for long periods of time without breakdown. The emulsion is ready for immediate use in a pretreatment tank upon heating to about ll lO-l20 F.

Molded polyolefin parts to be chemically plated are immersed in the foregoing pretreatment emulsion and held in the bath for to minutes. No special precleaning of the parts is necessary unless the surface is greasy or otherwise badly soiled. In such case any of the conventional proprietary cleanling compositions known in the art can be used. Upon withdrawing the parts from the pretreatment emulsion, they are immersed for 1 minute in a cold water rinse, placed in a nickel sulfate-ammonium hydroxide bath as before. A suitable composition here comprises about 18 grams per liter of nickel sulfate (hexahydrate) dissolved in equal volumes of water and concentrated (50 percent) ammonium hydroxide. Suitable sperating conditions are about 145 F. with an immersion period of about 15 minutes.

Following the phosphiding bath, the parts are again given a sold water rinse. They may then be transferred to a suitable electroless metal plating bath. A typical electroless nickel bath suitable for this purpose is described in US. Pat. No. 12,532,283, Example V, Table l. Thereafter the plated parts may be given an additional metal deposit in an electrolytic bath in conventional manner.

in place of the electroless nickel deposit, an electroless copper plate may be substituted at the corresponding step in the sequence of operations just described. A suitable electroless copper for this purpose is disclosed in US. Pat. No. 13,095,309, Example Again the parts may be further plated in a conventional electrolytic plating process to build up a lieavier deposit.

Complete coverage and excellent adhesion are obtained between substrates of polyethylene, polypropylene and polybutylene and the metal deposit using the foregoing procedure.

What is claimed is:

i. A process of pretreating the surface of molded polyolefin synthetic resins to improve adhesion thereto of a subsequently applied electrolessly deposited metal film, which comprises the step of contacting the surface of the molded resin with a stable oil-in-water emulsion of white phosphorus and trichloroethylene.

2. A process as defined in claim 1, wherein the polyolel'in is polypropylene.

3. A process as defined in claim 1 where the temperatures of the aqueous emulsion is 1l0-l20 F. and contact with the surface of the molded resin is maintained for about l5 minutes.

4. A process as defined in claim 1 wherein the trichloroethylene comprises about 15 percent by volume of the total emulsion and phosphorus is present to the limit of its solubility in the emulsion.

5. A process as defined in claim I, wherein said emulsion consists essentially of white phosphorus, trichloroethylene, water and surfactants in the proportion of 2.2 grams of white phosphorus, milliliters of trichloroethylene, milliliters of surfactant and 495 milliliters of water.

6. A process of plating a metal on a molded polyolefin substrate which comprises the steps of pretreating the surface of the molded polyolefin in accordance with the process defined in claim 1, rinsing the surface of the substrate in water, immersing the substrate in a phosphiding bath consisting of nickel sulfate and ammonium hydroxide, removing and rinsing the surface of the substrate in water and thereafter immersing the substrate in an electroless metal bath to deposit a metal film on the surface of the substrate.

llt I c t I! 

2. A process as defined in claim 1, wherein the polyolefin is polypropylene.
 3. A process as defined in claim 1 where the temperatures of the aqueous emulsion is 110*-120* F. and contact with the surface of the molded resin is maintained for about 15 minutes.
 4. A process as defined in claim 1 wherein the trichloroethylene comprises about 15 percent by volume of the total emulsion and phosphorus is present to the limit of its solubility in the emulsion.
 5. A process as defined in claim 1, wherein said emulsion consists essentially of white phosphorus, trichloroethylene, water and surfactants in the proportion of 2.2 grams of white phosphorus, 100 milliliters of trichloroethylene, 105 milliliters of surfactant and 495 milliliters of water.
 6. A process of plating a metal on a molded polyolefin substrate which comprises the steps of pretreating the surface of the molded polyolefin in accordance with the process defined in claim 1, rinsing the surface of the substrate in water, immersing the substrate in a phosphiding bath consisting of nickel sulfate and ammonium hydroxide, removing and rinsing the surface of the substrate in water and thereafter immersing the substrate in an electroless metal bath to deposit a metal film on the surface of the substrate. 